A wheel deceleration speed which is necessary for controlling an antilock brake system (hereinafter referred to as ABS) is easily obtainable with respect to a two-wheel drive automobile having non-driven wheels. However, in a four-wheel drive automobile, when a center differential and a rear-wheel shaft differential are locked and four wheels are directly connected with each other, all wheels are connected to an engine having a large moment of inertia and hence, a response speed of wheels for a change of a brake torque is remarkably lowered. Accordingly, it is hardly possible to obtain a wheel deceleration signal and hence, the wheel is locked. As one of means to overcome such a drawback, in general, a vehicle body acceleration sensor is mounted on the four-wheel drive automobile. The vehicle body acceleration in the advancing direction of the vehicle body is measured by such a vehicle body acceleration sensor and when the deceleration at the time of braking is smaller than a fixed value, a road surface is determined as a road surface having a low friction resistance value μ (hereinafter referred to as “low μ road surface”). Then, by changing over a control logic from a slip control logic for road surface having high friction resistance value μ (hereinafter referred to as “high μ road surface”) to a slip control logic for low μ road surface, the control accuracy is increased and hence, the locking of the wheel is prevented. Accordingly, the vehicle body acceleration sensor is indispensable particularly in the ABS of the four-wheel drive automobile and hence, when the vehicle body acceleration sensor is in trouble, the slip control becomes impossible and the wheel may be locked. In such circumstances, the detection of trouble of the vehicle body acceleration sensor is surely indispensable. Although several trouble modes are considered as the trouble mode of the vehicle body acceleration sensor, as one of such trouble modes, there has been known an output fixing trouble in which the outputting of a fixed vehicle body acceleration signal is maintained so that a vehicle body acceleration output signal is not changed in accordance with the acceleration of the vehicle body.
As one example of the related art which detects the output fixing trouble of the vehicle body acceleration sensor, there has been known a technique in which in a state that an antilock brake control is not performed, a vehicle body acceleration which is calculated based on a wheel speed sensor and a vehicle body acceleration which is detected by the vehicle body acceleration sensor are compared to each other, and when the difference between these vehicle body accelerations is equal to or more than a predetermined value, it is determined that the vehicle body acceleration sensor is in the output fixing trouble (see patent document 1, for example) Alternatively, there has been known a technique in which a vehicle body acceleration which acts on a vehicle body due to gravity which can be estimated when an automobile ascends or descends a slope having a sufficiently large inclination angle is set as a threshold value, and when a vehicle body acceleration which is calculated by a wheel speed sensor is larger than the threshold value and a vehicle body acceleration which is calculated by a vehicle body acceleration sensor is smaller than the threshold value, it is determined that the vehicle body acceleration sensor is in the output fixing trouble (for example, see patent document 2). Further, there has been known a technique in which a relative acceleration difference between a logic vehicle body acceleration which is calculated based on a wheel speed and a physical vehicle body acceleration which a vehicle body acceleration sensor detects is calculated, a continuation time in which the acceleration difference exceeds a preset value is measured, a minimum acceleration and a maximum acceleration of the physical vehicle body acceleration are stored during a state in which the acceleration difference exceeds the preset value, and at a point of time that the continuation time of the state in which the acceleration difference exceeds the preset value exceeds the preset time, when the difference between the stored minimum acceleration and the maximum acceleration is a value equal to or less than a fixed difference, it is determined that the vehicle body acceleration sensor is in the output fixing trouble (for example, see patent document 3).    Patent Document 1: JP-A-1-195168    Patent Document 2: JP-A-8-184610    Patent Document 3: JP-A-2003-63375